You might have better luck with a dirt/stone cannon. the main problem with the stone is how brittle it is. this would lead it to shatter after only a few shots, if that. if you packed the inside of your stone cannon with a very compact layer of dirt, or other dense padding, you might be able to re-use the cannon by refilling the cushion after every shot. the only downside is that you sacrifice a good deal of the force to do this, so your cannon will really only be effective against infantry, not fortifications.